A related lamp unit is described in Japanese Laid-Open Patent Application No. 2007-12368.
As shown in FIG. 9, the lamp unit 100 is constructed of a light source 102, a reflector 103 that surrounds the light source 102 from behind and reflects light emitted from the light source forwardly, a lamp housing 104 that receives the light source 102 and the reflector 103 therein, and a lamp lens 105 that closes an opening of the lamp housing 104. Further, the reflector 103 has a rib 107 that is formed in a rear side thereof. The rib 107 is capable of guiding ascending airflow produced in the lamp unit 100 by heat generation of the light source 102 and thermally convecting air. This can reduce possibility of generation of fog in the lamp unit 100.